1. Field of the Invention
The present invention relates generally to screw fasteners and more particularly to self-tapping screw fasteners having a novel thread design.
2. Description of the Prior Art
Many types of self-tapping screw fasteners such as wood screws and sheet metal screws have been available for a number of years. The screw fasteners are generally made from metal and include a cylindrical shank with an integrally joined external helical thread that forms a plurality of substantially uniformly spaced convolutions along the shank. The shank generally has a trailing end with a driving means, such as a tool engageable head and an entering end which may be blunt or pointed. Conventionally, the helical thread has a triangular thread profile which is determined by a transverse cross section of the screw thread. The thread thus presents a sharp edge on its crest so that axial loading of the screw fastener coupled with rotational motion drives the fastener into a workpiece and forms a complementary internal thread therein.
The method of use and the design of self-tapping screw fasteners are often adapted to accommodate the physical properties of the intended workpiece.
When the workpiece is a high strength but generally ductile material, such as most types of sheet metal, a pilot hole is generally provided in order that the torque necessary to drive the fastener be reduced. Pilot holes are also often provided when self-tapping screw fasteners are used in brittle materials, such as phenolic plastics, because tensile stresses generated by the fastener entering the workpiece frequently cause it to crack or become stripped of the complementary thread formed therein. When used in conjunction with a pilot hole, a blunt entering end on the screw fastener is generally sufficient to initiate proper entry into the workpiece.
When the workpiece is a relatively low strength material, such as many types of wood and various plastics, the fasteners are often driven without the benefit of a pilot hole. Screw fasteners for this purpose are usually provided with a conical or tapered entering end that often has a cutting edge to facilitate entry into the workpiece. The problems of cracking the workpiece and stripping the complementary thread are aggravated when the screw fasteners are used without benefit of a pilot hole.
In all cases, regardless of the workpiece material or the use of a pilot hole, it is desirable that the self-tapping screw fastener require only relatively low driving torque, and have high stripping torque and pullout strength and substantial resistance to loosening. It is also desirable to provide a fastener that will minimize the tendency to crack a brittle workpiece.
Various types of self-tapping screw fasteners have been developed in an effort to achieve at least some of the aforementioned desired characteristics. One such screw fastener is described in U.S. Pat. No. 3,207,023 and includes a first helical thread which has a generally triangular thread profile with a relatively large thread height and a relatively small included angle. The large thread height causes the thread to project into the workpiece relatively deeply so as to provide a large area of engagement between the thread and the workpiece. The small included angle of the thread profile facilitates entry of the high thread into the workpiece. A second helical thread is located between the convolutions of the first helical thread and also has a triangular thread profile but a relatively small thread height and large included angle. The second helical thread adds to the area of thread contact with the workpiece in an attempt to increase the resistance to loosening and the pullout strength of the screw fastener.
The screw fastener disclosed in U.S. Pat. No. 3,207,023 has the disadvantage that it requires two helical threads. This makes the fastener substantially more expensive and difficult to manufacture than conventional screw fasteners. It also requires that the screw fastener have a greater number of thread convolutions per inch, or a shorter pitch, in order to obtain the advantages of the invention. This, of course, increases the torque required to drive the screw fastener. Standard screw fasteners of this type generally have about 18 thread convolutions per inch.